Subsurface gas- and oil-shale samples of Texas shales from the Permian, Fort Worth, and Maverick basins and San Marcos Arch: core sampling for measured vitrintite-reflectance (Ro) determination: final technical summary report (FY 2010–2014)

Shale samples analyzed for measured vitrinite reflectance during FY 2010–2014 were collected from varying depositional basins in Texas and strata of different ages. They include the Upper Devonian and Lower Mississippian Woodford Shale (Permian Basin), the Lower Pennsylvanian Smithwick Shale (Fort Worth Basin), the Lower Permian shales and Spraberry Formation (Midland Basin), the Lower Cretaceous Pearsall Formation, and the Upper Cretaceous Eagle Ford Shale (Maverick Basin and adjacent area). Although an approximate trend of increasing vitrinite-reflectance values with depth (i.e., increasing thermal maturity, or rank) occurs in the Eagle Ford Shale of the San Marcos Arch, this pattern is not exhibited with the other units sampled. Moreover, when measured vitrinite-reflectance values are compared to calculated-Ro values of Lower Permian shales and the Spraberry Formation, consistently lower values occur with the measured-Ro data set. Sample values from the remaining three successions studied (with the possible exception of the Smithwick Shale) are characterized by similarly lower-than-expected vitrinite-reflectance values. These low values are probably a result of markedly lean successions and not of the presence of low-rank strata. (Lean = either no vitrinite was present in a sample or it was too small to be measured.) Oil- and gas-shale core samples do not appear to be ideal for measuring vitrinite reflectance primarily because of the fine-grained character of the rock, as opposed to coal, in which vitrinite is sufficiently coarse, visible, and abundant to consistently derive Ro values.Bureau of Economic Geolog

Regional Geology of the Low-Permeability Gas-Bearing Cleveland Formation, Western Anadarko Basin, Texas Panhandle: Lithologic and Depositional Facies, Structure, and Sequence Stratigraphy

The Upper Pennsylvanian (lower Missourian) Cleveland Formation produces gas from low-permeability ("tight") sandstone reservoirs in the western Anadarko Basin of the northeastern Texas Panhandle. In this six-county region, these reservoirs had produced more than 412 Bcf of natural gas through December 31, 1989. Because of their typically low permeability, the Cleveland sandstones require acidizing and hydraulic fracture treatment to produce gas at economic rates. Since 1982, the Gas Research Institute has supported geological investigations throughout the United States to develop the scientific and technological knowledge for producing from low-permeability, gas-bearing sandstones. As part of this program and the GRI Tight Gas Sands project, the Bureau of Economic Geology has been conducting research on low-permeability sandstones in the Cleveland Formation and on several other sandstone units of similar character in Texas and Wyoming. This effort is part of a broader program to increase the understanding and ultimate utilization of gas resources in these low-permeability formations through regional and field-specific geology, formation evaluation, and reservoir engineering. This report summarizes findings on the regional geology, depositional setting, sequence stratigraphy, and petrology of the Cleveland Formation. Geological research on the Cleveland began with an effort to choose a formation in which to drill Staged Field Experiment (SFE) well number 4, the latest in a series of SFE wells drilled since 1986 to conduct geological and engineering research on low-permeability gas reservoirs. Although the Cleveland Formation was not chosen for SFE No. 4, investigation of this low-permeability, gas-bearing sandstone continued with the drilling of cooperative wells in the unit. Because the Cleveland Formation contains an estimated 38 Tcf of gas in place, development of advanced technology and understanding that can be applied to this and other tight gas formations will have a positive impact on gas supply by improving gas recovery and lowering completion costs.Bureau of Economic Geolog

Geologic Challenges and Opportunites of the Cherokee Group Play (Pennsylvanian): Anadarko Basin, Oklahoma

The Middle Pennsylvanian Cherokee Group composes one of the most active natural gas plays in the Anadarko Basin of Oklahoma, having produced more than 1.2 Tcf from major (> 10 Bcf cumulative production) Cherokee reservoirs in Beckham, Custer, Roger Mills, and Washita Counties, the area currently experiencing the most active Cherokee development activity. Preliminary geologic study and telephone survey of 15 Cherokee operators satisfied three primary project objectives: (1) to summarize both the geologic characteristics of the Cherokee Group and the production highlights in the four-county area of current activity; (2) to summarize what current Cherokee producing companies perceive to be the primary geologic challenges they face in developing the Cherokee play; and (3) to suggest geologic strategies to help respond to these challenges. Geologic questions related to Cherokee gas-production enhancement are fundamental, and answers to these questions have the potential to alter current production strategies, reduce risk, and ultimately to increase natural gas reserves. Most of the surveyed Cherokee operators acknowledge that they have only a partial understanding of regional facies relations within the Cherokee depositional systems tracts. Moreover, there is no clear and integrated perspective of depositional systems, reservoir geometry, and diagenesis among all Cherokee fields in the play area. Reservoir geometry is complex and not readily predictable; therefore, drilling of infill wells, which characterizes the current development strategy of the Cherokee play, is fraught with uncertainty. The high degree of variation in porosity and permeability cannot be predicted from current knowledge of reservoir-quality patterns. A limited per-well drainage area suggests internal compartmentalization of sandstone reservoirs. Investigations at several scales can provide needed information. Improved and more precise modeling of (1) the regional spectra of Cherokee depositional settings at the play scale, (2) depositional facies and geometry at the field scale, and (3) facies architecture, diagenesis, and fracture distribution at the reservoir scale would aid the efficient exploitation of the remaining natural gas resources in the Cherokee play.Bureau of Economic Geolog

Subsurface Devonian and Mississippian Gas Shale Samples Barnett and Smithwick Shales (Fort Worth Basin) and Woodford and Barnett Shales (Delaware Basin) Core Sampling for Measured Vitrinite Reflectance Determination

This report summarizes activities carried out by the Bureau of Economic Geology (BEG) during Fiscal Year (FY) 2007 for the National Coal Resources Data System State Cooperative Program (NCRDS project). The report represents a departure from those prepared in previous years by providing a collection of Devonian-Mississippian and Mississippian gas-shale samples. Approval for this change in sample type was given by the USGS in March 2008. Samples were collected from whole cores of three wells in Wise and San Saba Counties, Fort Worth Basin, North Texas (Fig. 1), and Pecos County, Delaware Basin, West Texas (Fig. 2) for vitrinite-reflectance (Ro) analysis by the USGS. Other study activities include identification of the sampled shales' precise geographic location and their stratigraphic position.Bureau of Economic Geolog

Subsurface Oil-Shale Samples of the Upper Pennsylvanian Cline Shale, Midland Basin, West Texas: Core Sampling for Measured Vitrinite-Reflectance (Ro) Determination

This report summarizes activities carried out by the Bureau of Economic Geology (BEG) during fiscal year (FY) 2015 for the National Coal Resources Data System State Cooperative Program (NCRDS project). In a continuation of the sampling strategy for measured vitrinite-reflectance (Ro) determination initiated 6 years ago (Hentz and others, 2009) and conducted during the following five years (Hentz and others, 2010, 2011, 2012, 2014, 2015), this report provides a collection of oil-shale samples from the Upper Pennsylvanian Cline Shale of the Midland Basin in West Texas (Fig. 1).Bureau of Economic Geolog

Depositional and Structural Challenges of the Wilcox Lobo Natural Gas Trend, South Texas

To increase understanding and utilization of gas resources in the Wilcox Lobo play of South Texas, this report reviews current geological knowledge of the Lobo trend. An additional objective of this report is to identify areas where advancements in geological understanding could lead to substantial improvements in efficient development of the Wilcox Lobo trend natural gas resource. According to published accounts, Lobo sandstones formed in a variety of depositional environments in both shallow and deep marine waters. During and after deposition, the Lobo experienced repeated episodes of erosion, faulting, and diagenesis. Thus, accurate prediction of reservoir sandstone attributes is difficult, and this difficulty is cited by operators as a significant challenge to efficiently targeting the remaining gas resource. Knowledge that would aid in the emergence of this resource includes information on sandstone correlation and accurate zone identification, depositional systems and facies interpretations, controls on fault pattern variability, and, to a lesser extent, recognition of diagenetic patterns and faults and fractures that are below seismic resolution. Geologic challenges of the Lobo trend are opportunities for targeting increasingly smaller and more difficult-to-detect compartments with advanced technology.Bureau of Economic Geolog